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# Working principle of load cells

A load cell is a device that converts energy from one form to another. Specifically, the load cell is a force sensor that converts the kinetic energy of force (such as tension, compression, pressure, or torque) into a measurable electrical signal. The strength of the signal varies with the strength of the applied force. There are three basic types of load cells based on the output signal: hydraulic, pneumatic and strain gauge. #Weighing sensor#

The most commonly used type of weigher in industrial applications is the strain gauge load cell. The strain gauge load cell consists of a solid metal body (or "spring element") that fixes the strain gauge. The fuselage is usually made of aluminum, alloy steel or stainless steel, making it very strong, but with minimal flexibility. When applying weighing, the body of the load cell will deform slightly, but unless it is overloaded, it will always return to its original shape. In response to changes in body shape, strain gauges also change shape. This in turn causes the resistance of the strain gauge to change, which can then be measured as a voltage change. Since this change in output is proportional to the applied weight, the weight of the object can be determined from the voltage change.

Strain gauge load cell

Strain gauge load cell
How does the load cell work?

To answer "How does a load cell work?" We first need to know "How does a strain gauge work?" A strain gauge is a device that measures the resistance change when a force is applied. A typical strain gauge consists of very thin wires or foil nets, arranged in a grid pattern, and when strain is applied along one axis, a linear resistance change occurs. There are many types of strain gauges to choose from:

Linear strain gauge: The wire connected to the back of the strain gauge is parallel to the edge of the strain gauge. These are used to measure axial strain and bending strain. Shear strain gauges: the wires connected to the back of the strain gauges are arranged on both sides of the pressure frame in a 45o direction. These are used to measure shear strain. Strain gauges are usually used in conjunction with more strain gauges to improve accuracy. One active strain gauge is called a quarter bridge, two active strain gauges are a half bridge, and four active strain gauges are called a full bridge.

The resistance change of the strain gauge is different between the tension weighing cell and the compression weighing cell. The tension makes the strain gauge thinner and longer, increasing the resistance. The compressive force makes the strain gauge thicker and shorter, and the resistance decreases. The strain gauge is adhered to the thin back (bracket), which is directly connected to the weighing cell, so that the strain gauge can experience the strain of the load cell.

The resistance change measured by a single strain gauge is very small, about 0.12°. The sensitivity of the load cell increases with the number of strain gauges applied. A good way to turn these small changes into more measurable ways is to interconnect them as Wheatstone bridges.

Type of strain gauge

The strain gauges are arranged in different orientation patterns, depending on the type of force being measured. Bending strain, shear strain, axial strain, torque and pressure are all measured using a specific layout of strain gauges.

Wheatstone Bridge

The Wheatstone bridge is configured by four balancing resistors and a known excitation voltage is applied, as shown below:

Wheatstone Bridge
Before five is the known constant voltage and VO is measured. If all resistors are balanced, it means R1/R2 R3/R4 and then VO is zero. If one of the resistance values changes, there will be a resultant change in VO, which can be measured and explained using Ohm's law. Ohm's law states that the current (I, in amperes) that passes through a conductor between two points is proportional to the voltage (V) between the two points. Resistance (R, measured in ohms) is introduced as a constant in this relationship and has nothing to do with current. Ohm's law is expressed in the equation I = V/R.

When applied to 4 strips of the Wheatstone bridge circuit, the resulting equation is:

Wheatstone Bridge
In load cells, these resistors are replaced by strain gauges in alternating tension and compression measurements. When force is applied to the weighing cell, the resistance in each strain gauge changes and VO is measured. From the result data, VO can be easily determined using the above equation.